Tainted air Long-term exposure to pollution may speed up hardening of the arteries contributing to heart attacks and strokes, say US researchers.

The researchers, led by Dr Sara Adar from the University of Michigan and Dr Joel Kaufman from the University of Washington , examined how exposure to fine particle pollution affects the carotid arteries -- the arteries that supply the head, neck and brain with oxygenated blood.

They found higher concentrations of fine particulate matter (PM2.5) were linked to a faster rate of thickening, while reductions in exposure to fine matter particulates were linked to slower progression of the condition, they report in PLoS Medicine .

"Our findings help us to understand how it is that exposures to air pollution may cause the increases in heart attacks and strokes observed by other studies," says Adar.

Fine particulate matter (PM2.5) -- particles with a diameter of less than 2.5 micrometres or 1/30th the width of a human hair -- is mainly produced by motor vehicles, power plants, and other combustion sources.

It has been long been thought that there is a link between exposure to these particles and cardiovascular disease, but previous studies have proved inconclusive.

To examine the link in more detail, the researchers studied 5,362 people aged between 45 to 84 years old from six US metropolitan areas as part of the Multi-Ethnic Study of Atherosclerosis and Air Pollution (MESA Air).

At the beginning of the study, the participants had no signs of cardiovascular disease.

The researchers measaured air pollution near each person's house and performed two ultrasound measurements of the right carotid artery, separated by about three years.

After adjusting for other factors such as smoking, the authors found that on average, the thickness of the carotid vessel increased by 14 micrometres each year. The blood vessels of people exposed to higher levels of residential fine particulate air pollution, however, thickened faster than others living in the same metropolitan area.

"Linking these findings with other results from the same population suggests that persons living in a more polluted part of town may have a two per cent higher risk of stroke as compared to people in a less polluted part of the same metropolitan area," says Adar.

The researchers plan to follow the group for 10 years.

"If confirmed by future analyses of the full 10 years of follow-up in this cohort, these findings will help to explain associations between long-term PM2.5 concentrations and clinical cardiovascular events," they say.

Benefits of clean air

Associate Professor Adrian Barnett from the Institute of Health and Biomedical Innovation at the Queensland University of Technology says the "high-quality" study provides and important biological link between chronic exposure and cardiac health.

"Australians who work or live near major road junctions or factories are likely to have poorer health than if they lived or worked in a cleaner part of the city.

"On the positive side, the study demonstrates that if the air becomes cleaner over time then this creates health benefits.

"We would see health benefits in Australia if we were bold enough to implement policies that reduced traffic pollution," says Barnett.

Emeritus Professor Michael Moore, a toxicologist and former director of the National Research Centre for Environmental Toxicology at the University of Queensland, agrees the study provides further evidence of the health impacts of poor air quality.

But he says the association between particulates and carotid artery thickness cannot prove that PM2.5 is the causative factor.

"PM2.5 may well be an indicator of other components of pollution such as nitrogen oxides, carbon monoxide or ozone. It may also be a surrogate for ultra-fine particulates," says Moore.

"Future studies of concurrent pollution factors will resolve the likely mechanistic rationale for these effects."